The present invention relates to an acrylic rubber-based composition or, more particularly, to an acrylic rubber-based composition suitable for hot-air vulcanization under normal pressure in a continuous molding and vulcanizing process with a greatly improved productivity as compared with conventional acrylic rubber-based compositions which can be molded and vulcanized only under compression in a batch-wise process.
As is well known, acrylic rubbers are widely used as a principal material of rubber parts in automobiles since these rubbers have outstandingly excellent high heat resistance and oil resistance among various synthetic rubbers. It is known that the velocity of vulcanization of acrylic rubbers in vulcanization largely depends on the types of the functional groups in the rubber molecules to pertain to the vulcanization reaction. For example, the vulcanizing velocity of an acrylic rubber having active halogen atoms as the vulcanization sites is large as compared with acrylic rubbers functional with epoxy groups. While the vulcanizing velocity of an acrylic rubber should desirably be as high as possible since a rubber composition contains some vaporizable ingredients and the vulcanization reaction usually produces a volatile matter as a decomposition product which unavoidably causes foaming or surface blistering of the shaped and vulcanized rubber article unless the vulcanization is undertaken under compression, the vulcanizing velocity of the active halogen-functional acrylic rubber composition is still not adequately high. Therefore, such an acrylic rubber composition is not suitable for the so-called hot-air vulcanization under normal pressure in order to obtain shaped and vulcanized rubber articles free from foaming and blistering, which can be obtained only by the vulcanization under compression.
Japanese Patent Publications No. 2-1859 and No. 4-30963 propose a rapid vulcanization-type acrylic rubber composition by introducing organosilicon groups having a silicon-bonded vinyl group into the acrylic rubber molecules. A substantial improvement can be obtained by this means relating to the vulcanizing velocity but the improvement obtained thereby is far from satisfactory because microscopic bubbles are always found in the vulcanizate obtained by the hot-air vulcanization under normal pressure. Moreover, the acrylic rubber composition of this type has another problem that the vulcanization reaction thereof is subject to inhibition by the atmospheric oxygen so that the vulcanization in the surface layer is more or less incomplete to cause falling of the surface layer when scratched with a finger nail. This problem of vulcanization inhibition by the atmospheric oxygen can of course be avoided and full vulcanization can be obtained even in the surface layer of the rubber article by conducting the vulcanization under an atmosphere of high temperature nitrogen gas. However, but this measure is not practicable because leakage of nitrogen gas is more or less unavoidable from the vulcanization ovens working under nitrogen atmosphere possibly causing a workers safety problem due to oxygen deficiency in the working environment which can be solved only with great expenses for the construction and maintenance of the facilities.